…ersistent (#2781). Principle III.

Settings → FlexControl, Settings → USB Cables, and the XVTR overlay button each
called `new RadioSetupDialog(...)` directly, bypassing the m_persistentDialogs
registry that PR #2772 (RadioSetupDialog → PersistentDialog migration) set up.
Symptoms: clicking any two entries opens two windows, frameless toggle skips
the shortcut-opened dialog, and "first opener wins" signal wiring diverges.

This pass routes all four entry points (primary Radio Setup, FlexControl, USB
Cables, XVTR overlay) through showOrRaisePersistent + selectTab, and factors
the txBandSettingsRequested / serialSettingsChanged / sliceLetterDisplayModeChanged
/ QDialog::finished wiring into a single private helper wireRadioSetupDialogSignals
so a fresh construction picks up the full handler set regardless of which menu
clicked first.  The dialog now lives in m_persistentDialogs unconditionally, so
the special-case qobject_cast in setFramelessWindow is removed — the
persistentDialogs loop below already handles it.

Also re-asserts the primary Radio Setup entry's #2772 migration that #2786
silently reverted during a panadapter-layout merge.

Blast radius (codegraph impact on MainWindow::buildMenuBar, max_depth=3):
risk_score=0.7297, 18 high_risk affected (top: MainWindow::MainWindow,
AppSettings::instance, AppSettings::value).  The score reflects buildMenuBar's
breadth across the menu surface; the actual edit is four lambda bodies plus a
helper, none of which alter buildMenuBar's caller-facing contract.  No new
test-target link dependencies (wireRadioSetupDialogSignals is private to
MainWindow.cpp, RadioSetupDialog.h was already #included).